Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 61: Glucose-6-phosphate dehydrogenase (G6PD) is primarily found in which cellular compartment?

A. Mitochondria
B. Cytosol (Correct Answer)
C. Golgi apparatus
D. Endoplasmic reticulum

Explanation: **Explanation:** **1. Why Cytosol is Correct:** Glucose-6-phosphate dehydrogenase (G6PD) is the **rate-limiting enzyme** of the **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway). The HMP shunt is a unique pathway because it does not involve the mitochondria and occurs entirely within the **cytosol** of the cell. This location is strategic, as the pathway generates **NADPH**, which is required for cytosolic processes such as fatty acid synthesis, steroid synthesis, and the reduction of glutathione to protect cells against oxidative stress. **2. Why Other Options are Incorrect:** * **Mitochondria:** This compartment houses the TCA cycle, Beta-oxidation of fatty acids, and the Electron Transport Chain (ETC). G6PD is not involved in these oxidative phosphorylation processes. * **Golgi Apparatus:** This organelle is primarily involved in the post-translational modification, sorting, and packaging of proteins; it does not host the enzymes of primary carbohydrate metabolism. * **Endoplasmic Reticulum (ER):** While the ER is involved in protein synthesis (Rough ER) and lipid/steroid synthesis (Smooth ER), the initial enzymatic steps of the HMP shunt occur in the surrounding cytosol. (Note: Glucose-6-phosphatase is located in the ER, but G6PD is cytosolic). **3. Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. It leads to **hemolytic anemia** triggered by oxidative stress (e.g., Fava beans, Primaquine, Infection) because RBCs lack mitochondria and rely solely on the HMP shunt for NADPH to maintain reduced glutathione. * **Tissue Distribution:** G6PD activity is highest in tissues requiring NADPH for reductive biosynthesis (Liver, Adipose tissue, Lactating mammary glands, Adrenal cortex) and in RBCs for antioxidant defense. * **Key Product:** The HMP shunt produces **Ribose-5-phosphate** (for nucleotide synthesis) and **NADPH** (not NADH).

Question 62: The oxidative phase of the HMP shunt pathway is least active in which of the following tissues?

A. Adrenal cortex
B. Lactating mammary gland
C. Red blood cells
D. Skeletal muscle (Correct Answer)

Explanation: **Explanation:** The oxidative phase of the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway) is primarily responsible for the production of **NADPH**. Therefore, this pathway is most active in tissues that require high amounts of NADPH for reductive biosynthesis or protection against oxidative stress. **Why Skeletal Muscle is the Correct Answer:** Skeletal muscle lacks the enzymes for significant fatty acid or steroid synthesis. Its primary metabolic requirement is ATP production via glycolysis and the TCA cycle. Since it does not perform large-scale reductive biosynthesis, the activity of Glucose-6-Phosphate Dehydrogenase (G6PD)—the rate-limiting enzyme of the oxidative phase—is extremely low in resting skeletal muscle. **Analysis of Incorrect Options:** * **Adrenal Cortex:** Highly active in the HMP shunt because NADPH is essential for the hydroxylation reactions involved in **steroid hormone synthesis**. * **Lactating Mammary Gland:** Requires massive amounts of NADPH for the **de novo synthesis of fatty acids** found in breast milk. * **Red Blood Cells (RBCs):** Utilize NADPH to maintain a pool of **reduced glutathione**, which is critical for neutralizing reactive oxygen species (ROS) and preventing hemolysis. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** Glucose-6-Phosphate Dehydrogenase (G6PD), regulated by NADP+ levels. * **Key Products:** NADPH (for biosynthesis/antioxidant defense) and Ribose-5-phosphate (for nucleotide synthesis). * **Tissues with High HMP Activity:** Liver, Adrenal cortex, Testes/Ovaries, Lactating mammary gland, RBCs, and Phagocytic cells (for respiratory burst). * **Clinical Correlation:** G6PD deficiency leads to hemolytic anemia under oxidative stress (e.g., fava beans, primaquine) because RBCs cannot generate enough NADPH to maintain glutathione in its reduced state.

Question 63: A 30-year-old male has excessive urinary glucose levels, but his blood glucose levels are normal. Investigations reveal high levels of L-xylulose in urine. A genetic defect in which of the following pathways is most likely responsible?

A. Tricarboxylic acid cycle
B. Uronic acid pathway (Correct Answer)
C. Glycolysis
D. Hexose monophosphate shunt

Explanation: ### Explanation The clinical presentation described is **Essential Pentosuria**, a rare, benign autosomal recessive condition. **1. Why the Uronic Acid Pathway is correct:** The Uronic Acid pathway is responsible for the conversion of glucose to glucuronic acid, ascorbic acid (in most mammals, but not humans), and pentoses. In this pathway, **L-xylulose** is normally reduced to **xylitol** by the enzyme **L-xylulose reductase** (using NADPH). In individuals with Essential Pentosuria, there is a genetic deficiency of this enzyme. This leads to the accumulation of L-xylulose in the blood, which is subsequently excreted in the urine. Since this pathway is independent of insulin and does not involve glucose-6-phosphatase, blood glucose levels remain normal, and there are no symptoms of diabetes mellitus. **2. Why the other options are incorrect:** * **Tricarboxylic acid (TCA) cycle:** This is the final common pathway for the oxidation of carbohydrates, fats, and proteins. Defects here usually lead to severe metabolic acidosis or neurological issues, not isolated pentosuria. * **Glycolysis:** This pathway converts glucose to pyruvate. Defects (like Pyruvate Kinase deficiency) typically manifest as hemolytic anemia. * **Hexose monophosphate (HMP) shunt:** While this pathway produces D-ribose and NADPH, it is not the source of L-xylulose. A defect here (like G6PD deficiency) leads to hemolysis under oxidative stress. **3. High-Yield Clinical Pearls for NEET-PG:** * **Essential Pentosuria** is one of the "Inborn Errors of Metabolism" originally described by Archibald Garrod. * **Diagnostic Clue:** The urine gives a positive **Benedict’s test** (reducing sugar) but a negative **Glucose Oxidase test** (specific for glucose). * **Key Enzyme:** L-xylulose reductase (also known as Dicarboxylate reductase). * **Drug Interaction:** Administration of drugs like **Aminopyrine** or **Barbiturates** can increase the excretion of L-xylulose in these patients as they stimulate the uronic acid pathway.

Question 64: What is the primary action of epinephrine on the liver?

A. Glycolysis
B. Lipolysis
C. Glycogenolysis (Correct Answer)
D. Gluconeogenesis

Explanation: ### Explanation **Primary Action: Glycogenolysis** Epinephrine (adrenaline) is a "fight-or-flight" hormone that aims to rapidly increase blood glucose levels to provide energy for muscles and the brain. In the liver, epinephrine binds primarily to **$\beta_2$-adrenergic receptors**, triggering the Adenylyl Cyclase-cAMP pathway. This activates **Protein Kinase A (PKA)**, which phosphorylates and activates **Glycogen Phosphorylase**, the rate-limiting enzyme of glycogenolysis. This leads to the rapid breakdown of glycogen into glucose-1-phosphate, which is then converted to free glucose and released into the bloodstream. **Analysis of Incorrect Options:** * **A. Glycolysis:** Epinephrine actually **inhibits** glycolysis in the liver. By increasing cAMP levels, it inhibits Phosphofructokinase-1 (PFK-1) indirectly, ensuring that the glucose produced is spared for peripheral tissues rather than being consumed by the liver itself. * **B. Lipolysis:** While epinephrine does stimulate lipolysis, this occurs primarily in **adipose tissue** (via hormone-sensitive lipase), not the liver. * **D. Gluconeogenesis:** Epinephrine does stimulate gluconeogenesis in the liver, but it is considered a **secondary or delayed action** compared to the near-instantaneous activation of glycogenolysis. Glycogenolysis is the "primary" and fastest response to acute stress. **High-Yield NEET-PG Pearls:** * **Dual Receptor Action:** In the liver, epinephrine can also bind to **$\alpha_1$ receptors**, which increases intracellular **Calcium ($Ca^{2+}$)**. This also activates glycogen phosphorylase via the Calmodulin complex, independent of cAMP. * **Muscle vs. Liver:** In skeletal muscle, epinephrine stimulates glycogenolysis to provide ATP for contraction, but because muscle lacks **Glucose-6-Phosphatase**, the resulting glucose cannot be released into the blood; it enters glycolysis instead. * **Key Enzyme:** Remember that **Glycogen Phosphorylase** is active in its phosphorylated form (*Phosphorylase a*).

Question 65: NADPH is generated by the action of which enzyme?

A. Glucose 6 phosphate dehydrogenase (Correct Answer)
B. Glucose 1 phosphate dehydrogenase
C. Glucose 1,6 diphosphate dehydrogenase
D. All of the above

Explanation: **Explanation:** The correct answer is **Glucose 6-phosphate dehydrogenase (G6PD)**. **1. Why G6PD is correct:** G6PD is the rate-limiting and regulatory enzyme of the **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway). This pathway occurs in the cytosol and is the primary source of **NADPH** in the body. G6PD catalyzes the oxidation of Glucose 6-phosphate to 6-phosphogluconolactone, simultaneously reducing $NADP^+$ to $NADPH$. This $NADPH$ is essential for reductive biosynthesis (e.g., fatty acids, steroids) and for maintaining the pool of reduced glutathione to protect cells against oxidative stress. **2. Why other options are incorrect:** * **Glucose 1-phosphate dehydrogenase:** This enzyme does not exist in human carbohydrate metabolism. Glucose 1-phosphate is an intermediate in glycogenesis and glycogenolysis, managed by enzymes like phosphoglucomutase. * **Glucose 1,6-diphosphate dehydrogenase:** This is not a recognized enzyme in the HMP shunt or major metabolic pathways. Glucose 1,6-bisphosphate acts primarily as a cofactor for the enzyme phosphoglucomutase. **Clinical Pearls for NEET-PG:** * **G6PD Deficiency:** The most common enzymopathy worldwide. Deficiency leads to decreased NADPH, causing oxidative damage to RBCs, resulting in **Heinz bodies**, **Bite cells**, and acute hemolytic anemia (often triggered by Fava beans, infections, or drugs like Primaquine). * **Tissues involved:** The HMP shunt is highly active in the liver, lactating mammary glands, adrenal cortex (for steroid synthesis), and RBCs (for antioxidant defense). * **Transketolase:** Another key HMP shunt enzyme; it requires **Thiamine (B1)** as a cofactor. Measuring its activity is used to diagnose Thiamine deficiency.

Question 66: A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

A. Liver (Correct Answer)
B. Heart
C. Skeletal muscle
D. Lysosome

Explanation: **Explanation:** The maintenance of blood glucose during fasting or starvation is primarily the responsibility of the **Liver**. In this patient, who has been unable to eat for 3 days, the body has transitioned from the post-absorptive state to the early starvation phase. **Why the Liver is Correct:** The liver is the central organ for glucose homeostasis. It maintains blood glucose through two key pathways: 1. **Glycogenolysis:** The breakdown of stored glycogen (exhausted within 12–24 hours of fasting). 2. **Gluconeogenesis:** The synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, and glucogenic amino acids). By day 3 of starvation, gluconeogenesis in the liver is the **primary source** of blood glucose to support glucose-dependent tissues like the brain and RBCs. The liver uniquely possesses the enzyme **Glucose-6-Phosphatase**, allowing it to release free glucose into the bloodstream. **Why Other Options are Incorrect:** * **Heart:** The heart is a consumer of energy, not a producer. It primarily utilizes fatty acids and ketone bodies during starvation to spare glucose for the brain. * **Skeletal Muscle:** While muscle stores significant glycogen, it lacks **Glucose-6-Phosphatase**. Therefore, muscle glycogen can only be used locally for energy and cannot be released as glucose into the blood. * **Lysosome:** These are organelles involved in cellular degradation (autophagy) and do not play a direct role in systemic blood glucose regulation. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Glucose-6-Phosphatase is the "marker enzyme" for gluconeogenesis, found in the liver and kidney cortex, but absent in muscle. * **Timeline:** Liver glycogen is depleted in ~18 hours; thereafter, gluconeogenesis becomes the sole source of blood glucose. * **Kidney Role:** In prolonged starvation (>5–6 days), the kidney cortex also contributes significantly (up to 40%) to gluconeogenesis.

Question 67: All of the following are intermediates of the TCA cycle, except?

A. Malonate (Correct Answer)
B. alpha-ketoglutarate
C. Succinate
D. Fumarate

Explanation: **Explanation:** The **Tricarboxylic Acid (TCA) cycle**, also known as the Krebs cycle, occurs in the mitochondrial matrix and is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. **Why Malonate is the Correct Answer:** **Malonate** is not an intermediate of the TCA cycle; rather, it is a potent **competitive inhibitor** of the enzyme **Succinate Dehydrogenase** (Complex II). It is a structural analogue of Succinate. By binding to the active site of the enzyme, malonate prevents the conversion of succinate to fumarate, effectively halting the cycle. This is a classic example of competitive inhibition frequently tested in biochemistry. **Analysis of Incorrect Options:** * **Alpha-ketoglutarate:** Formed from Isocitrate by *Isocitrate Dehydrogenase*. It is a key rate-limiting step and a precursor for glutamate synthesis. * **Succinate:** Formed from Succinyl-CoA by *Succinyl-CoA Synthetase* via substrate-level phosphorylation (producing GTP). * **Fumarate:** Formed from the oxidation of Succinate by *Succinate Dehydrogenase*. It is also a link to the Urea cycle and Tyrosine catabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Malonate vs. Malate:** Do not confuse the two. **Malate** is a TCA intermediate (formed from fumarate), while **Malonate** is the inhibitor. * **Succinate Dehydrogenase:** It is the only enzyme of the TCA cycle that is **integral to the inner mitochondrial membrane** (part of the Electron Transport Chain as Complex II). * **Fluoroacetate:** Another TCA inhibitor (found in some plants) that inhibits *Aconitase* after being converted to Fluorocitrate ("Suicide inhibition").

Question 68: Glucuronic acid and Iduronic acid are:

A. Anomers
B. Enantiomers
C. Functional isomers
D. Epimers (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Epimers** **Underlying Concept:** Epimers are stereoisomers that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). Glucuronic acid and Iduronic acid are both uronic acids derived from glucose. They are **C-5 epimers**. * In **D-Glucuronic acid**, the carboxyl group (-COOH) at C-5 is positioned above the ring (in the D-configuration). * In **L-Iduronic acid**, the configuration at C-5 is inverted. **Why other options are incorrect:** * **Anomers (A):** These differ only at the hemiacetal/hemiketal carbon (C-1 for glucose). Glucuronic and Iduronic acids differ at C-5. * **Enantiomers (B):** These are non-superimposable mirror images where every chiral center is inverted (e.g., D-Glucose and L-Glucose). These two acids are not mirror images. * **Functional Isomers (C):** These have the same molecular formula but different functional groups (e.g., Glucose and Fructose). Both molecules here are uronic acids. **Clinical Pearls for NEET-PG:** 1. **GAG Composition:** Iduronic acid is a major component of Glycosaminoglycans (GAGs) like **Heparin** and **Dermatan sulfate**. 2. **Epimerization:** In the body, D-glucuronic acid residues in heparin chains are converted to L-iduronic acid by the enzyme **uronyl C-5 epimerase**. 3. **Other High-Yield Epimers:** * **Glucose and Galactose:** C-4 epimers. * **Glucose and Mannose:** C-2 epimers. * **Ribulose and Xylulose:** C-3 epimers. 4. **Glucuronic Acid Function:** It is essential for the **conjugation of bilirubin** and the detoxification of xenobiotics in the liver.

Question 69: What is the amphibolic cycle?

A. Citric acid cycle (Correct Answer)
B. Glycolysis
C. Protein synthesis
D. Lipolysis

Explanation: **Explanation:** The **Citric Acid Cycle (TCA cycle or Krebs cycle)** is termed an **amphibolic cycle** because it plays a dual role in metabolism, involving both **catabolic** (breakdown) and **anabolic** (synthetic) pathways. 1. **Catabolic Role:** It is the final common oxidative pathway for carbohydrates, fats, and amino acids, where Acetyl-CoA is oxidized to $CO_2$ and $H_2O$ to generate energy (ATP, NADH, $FADH_2$). 2. **Anabolic Role:** Intermediates of the cycle serve as precursors for various biosynthetic pathways. For example: * **Succinyl-CoA** is used for Heme synthesis. * **$\alpha$-Ketoglutarate** and **Oxaloacetate** are used for synthesis of non-essential amino acids (via transamination). * **Citrate** is exported to the cytosol for fatty acid and cholesterol synthesis. **Why other options are incorrect:** * **Glycolysis:** Primarily a **catabolic** pathway that breaks down glucose into pyruvate. While some intermediates can divert to other pathways, its primary physiological role is energy production. * **Protein Synthesis:** A purely **anabolic** process (building proteins from amino acids). * **Lipolysis:** A purely **catabolic** process (breakdown of lipids into fatty acids and glycerol). **High-Yield NEET-PG Pearls:** * **Anaplerotic Reactions:** Since TCA intermediates are consumed for biosynthesis, they must be replenished. The most important anaplerotic reaction is the conversion of **Pyruvate to Oxaloacetate** by *Pyruvate Carboxylase* (requires Biotin). * **Location:** The TCA cycle occurs entirely in the **mitochondrial matrix**. * **Key Regulatory Enzyme:** Isocitrate Dehydrogenase (rate-limiting step).

Question 70: McArdle's disease is due to deficiency of which enzyme?

A. Myophosphorylase (Correct Answer)
B. Liver phosphorylase
C. Glucose-6-phosphatase
D. Acid maltase

Explanation: **Explanation:** McArdle’s disease, also known as **GSD Type V**, is an autosomal recessive disorder characterized by the deficiency of **Myophosphorylase** (muscle glycogen phosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during strenuous exercise, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Analysis of Options:** * **A. Myophosphorylase (Correct):** This is the muscle-specific isoform of glycogen phosphorylase. Its absence prevents the breakdown of muscle glycogen. * **B. Liver phosphorylase:** Deficiency of this enzyme leads to **Hers disease (GSD Type VI)**, which primarily presents with hepatomegaly and mild hypoglycemia, rather than muscle symptoms. * **C. Glucose-6-phosphatase:** Deficiency leads to **Von Gierke’s disease (GSD Type I)**, the most common GSD, characterized by severe fasting hypoglycemia, lactic acidosis, and "doll-like" facies. * **D. Acid maltase (α-1,4-glucosidase):** Deficiency leads to **Pompe disease (GSD Type II)**, which affects the heart and muscles due to lysosomal glycogen accumulation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ischemic Forearm Exercise Test:** Patients with McArdle’s show a **failure of blood lactate to rise** after exercise (since glycogen cannot be converted to lactate), while ammonia levels rise significantly. 2. **Second Wind Phenomenon:** Patients often experience a sudden improvement in exercise tolerance after 10–15 minutes as the body switches to using free fatty acids and blood glucose for energy. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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